CONDUCTIVITY, in heat, is the quantity of heat (q.v.) passing per second through a slab of unit cross-sectional area when the temperature gradient between the two faces is unity. Electrical conductivity is the current, or the quantity of electricity (q.v.) passing per second, through a similar slab when the po tential gradient is unity, and it is the reciprocal of the resistivity. In sound (q.v.) the conductivity of the orifice or neck of a resona tor is the ratio of the area to the length of the orifice.